Process for cylinder-selective leakage testing of the combustion chambers in a combustion engine

ABSTRACT

The invention concerns a process for the cylinder-selective leakage testing of the combustion chambers in a combustion engine during engine operation, but with the combustion process being suppressed, wherein for each cylinder, from the course of an engine shaft speed value, engine shaft speed differentials are determined to which a value for the compression pressure is allocated.

BACKGROUND OF THE INVENTION

[0001] The invention concerns a process for the cylinder-selective leakage testing of the combustion chambers in combustion engines during engine operation, but with the combustion process being suppressed, as known from DE 197 34 680 A1.

[0002] Engine operation with a suppressed combustion process occurs for instance when the combustion engine is started by means of an electric starter without any fuel being supplied and without any external ignition, or when a motor vehicle rolls on a downhill gradient whilst the engine transmission is engaged and the overrun fuel cut-off is activated.

[0003] A device for evaluating the compression of a multi-cylinder combustion engine by means of a type generic process is known for example from DE 43 37 720 A1. With this process, a signal is evaluated which is proportional to the starter current during combustion engine start-up. Here, the significant increase of the required starter current for gas mixture compression during the compression stroke of a cylinder is evaluated. If a cylinder shows a leakage of any kind, this can be detected by measuring the starter current that should normally be applied.

[0004] This method of leakage testing the combustion chambers is meaningful for combustion engines with electric starters which are not equipped with the means for detecting the crankshaft rotation angle and determining the current crankshaft speed.

[0005] The disadvantage of this process is that the leaking testing of the combustion chambers can be effected only on compression of the gas mixture during the compression stroke of the combustion engine cylinders. Moreover, the components required for measuring the starter current can be used for leakage testing only; therefore, this method of leakage testing is very costly and expensive.

[0006] From the type-establishing DE 197 34 680 A1, there is known a process for the cylinder-selective leakage testing of the combustion chambers in combustion engines by means of detecting the crankshaft rotation angle and determining the current crankshaft speed during motor operation with a suppressed combustion process; with this process, characteristic values are derived and obtained, during the compression and/or combustion strokes of the combustion engine cylinders, from the characteristic curves of the current crankshaft speeds; these characteristic values are correlated to the actual compression pressure in the cylinder combustion chambers, thus providing for leakage defects in the combustion chambers of the combustion engine to be detected cylinder-selectively.

[0007] The disadvantage of this process is that the statement on the cylinder-selective compression pressure can be made only by comparing the cylinders against each other. In so doing, this statement will be falsified by unavoidable cross-influencing of all characteristic values among the combustion chambers. Such cross-influencing occurs for example due to the fact that in the event of a combustion chamber having a leakage the rotation of the crankshaft is retarded less so that the combustion chamber which subsequently is to be compressed is actually compressed at a higher crankshaft speed. This makes it impossible to assess the cause of a combustion chamber leakage. In addition, this process cannot be used for combustion engines with just a single combustion chamber.

SUMMARY OF THE INVENTION

[0008] The invention is based on the task to state an improved process for the leakage testing of the combustion chambers in combustion engines by means of which process the compression pressure within the combustion chambers can be determined directly, with any cross-influences among the combustion chambers being compensated, and which process can also be used for combustion engines featuring just a single combustion chamber.

[0009] According to the invention, this task is solved by the characterizing features of patent claim 1. In so doing, current engine shaft speeds are determined from the engine shaft rotation angles for each cylinder of the combustion engine. Here, engine shaft rotation is subdivided into a limited number of angle segments, which allows a number of engine shaft speeds corresponding to the number of angle segments to be determined. The crankshaft speed will then change only from one angle segment to the next, within an angle segment the engine shaft speed will be assumed to be constant.

[0010] Next, engine shaft speed differentials will be determined for each cylinder from the course of engine shaft speeds. For this, it is advantageous to use the increase in engine shaft speed following movement through the top dead center of a cylinder.

[0011] These engine shaft speed differentials are each allocated a characteristic value which is preferably formed respectively as a percentage share of the engine shaft speed differential of a cylinder relative to the highest determined engine shaft speed differential of all cylinders.

[0012] These characteristic values are allocated the value of the compression pressure, with the maximum compression pressure corresponding to the nominal compression ratio under normal conditions.

[0013] The compression pressures determined for each cylinder of the combustion engine are compared with a specified threshold value, if necessary with a cylinder-specifically specified threshold value, with a fault signal being output if the actual value falls below the threshold value.

[0014] Before leakage testing of the combustion chambers in the combustion engine is carried out, it is intended that a test for compliance with the constraints prerequisite for any leakage testing is carried out. Such a constraint is the minimum engine shaft speed, for instance, from which the compression pressure can be determined reliably from an engine shaft speed differential.

[0015] The engine shaft speed differential of a cylinder is determined by determining the value of the maximum current engine shaft speed differential after the top dead center of the cylinder has been moved through minus the actual current engine shaft speed directly following the top dead center of the same cylinder of the combustion engine.

[0016] In order to improve evaluation precision, the engine shaft speed differentials of the cylinders are corrected relative to these mean engine shaft speeds of the rotation angle segments in accordance with characteristic curves/characteristic fields or an algorithm that can all be applied.

[0017] In addition, the cylinder-specific engine shaft speed differentials are corrected relative to the engine temperature by means of characteristic curves that can be applied.

[0018] In the case of combustion engines with multiple cylinders the engine shaft speed differential for the next cylinder in the ignition sequence is corrected in relation to the engine shaft speed differential of the last cylinder in the ignition sequence.

[0019] A further development of the invention provides for the compression pressures of the cylinders and/or the cylinder-selective engine shaft speed differentials and/or the characteristic values to be stored for comparative purposes after the combustion engine has been manufactured, repaired, or at any other intervals required.

[0020] Another further development of the invention provides for irregularities of the compression pressures of the combustion chambers in the combustion engine to be displayed e.g. on the instrument panel of a motor vehicle.

[0021] In order to condition the combustion engine it is provided that, before starting the detection of the current engine shaft speeds, the combustion engine runs through e.g. a working cycle corresponding to 720° in crankshaft rotation.

[0022] For example, the crankshaft speed, the camshaft speed or the starter shaft speed can be used as engine shaft speeds.

[0023] A final development of the invention provides for the leakage test process to be used in combustion engines featuring just a single cylinder, and with the engine shaft speed differential being compared with a stored engine shaft speed differential.

[0024] In the following, and in connection with two drawings, the process for cylinder-selective leakage testing will be illustrated and explained using an eight cylinder combustion engine as an example, and in respect of which the crankshaft speed is determined as the engine shaft speed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The figures below show

[0026]FIG. 1 a typical course of the curve for the current crankshaft speeds of the eight cylinder combustion engine over a 720 degree crankshaft rotation angle, with cylinder 1 showing a leakage.

[0027]FIG. 2 a flow chart for the execution of the cylinder-selective compression detection.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] The cylinder-selective leakage testing is effected when starting the combustion engine, with fuel supply and ignition remaining suppressed, and with an electric motor accelerating the crankshaft speed n to an actual value between 150 and 250 revolutions per minute. In the case of a combustion engine operating according to the four-stroke principle, the variation of the crankshaft speed n results from the compression stroke during which a piston will compress the gas mixture contained in the combustion chambers, with closed gas shuttle valves, down to approx. one tenth of the volume. This causes the crankshaft speed n to decrease up to the top dead center, and to increase again thereafter due to the gas mixture expanding again.

[0029] For determining the current crankshaft speed n, the crankshaft is equipped with a measuring device and an associated control unit. The measuring device consists of a sensor wheel revolving together with the crankshaft, provided with 36 markings and an additional marking, which are scanned by an induction sensor. From the signals of the induction sensor, the control unit will determine 36 current crankshaft speeds n during a single crankshaft revolution. The additional marking characterizes a crankshaft angle position, known to the control unit, within the working cycle x of the combustion engine, e.g. the top dead center of cylinder i=1.

[0030] The markings subdivide a single crankshaft rotation into 36 equal segments, with the induction sensor detecting the interval between two segments. For the control unit, the crankshaft speed n will thus change only from one segment to the next segment, within a segment the crankshaft speed n will be assumed to be constant. The control unit is thus provided with the information on crankshaft speed n and the crankshaft rotation angle at a resolution of 10 degrees.

[0031] For the clear allocation of the periodically repeating working cycle x—which comprises two crankshaft rotations—of a combustion engine relative to the crankshaft rotation angle, the camshaft is equipped with a further measuring device enabling detection of the camshaft rotation angle.

[0032] The signal sensor of the further measuring device of the camshaft consists of a sensor wheel revolving together with the camshaft, provided with 12 markings and an additional marking, which are also scanned by an induction sensor. The additional marking characterizes a camshaft angle position known to the control unit. From the signals of this induction sensor, the control unit will thus be able to determine the camshaft speed and the camshaft rotation angle at a resolution of 30 degrees.

[0033] The measuring device of the crankshaft and the further measuring device of the camshaft enable the control unit to allocate an event within the working cycle x of the combustion engine, which repeats periodically every two crankshaft rotations, to a change in the current crankshaft speed n. For example, the control unit can allocate an increase of the crankshaft speed n to the expansion of the cylinder i=3.

[0034] The automatic compression detection AVD of the combustion chambers need not necessarily occur at each start-up of the combustion engine. The automatic compression detection can be carried out, for instance, when a preset service life of the combustion engine has been reached, following fixed time intervals, or after it has been initiated manually. In order to carry out the automatic compression detection, flag <AVD-Start> will be set in the control unit; this will cause the flow chart shown in FIG. 2 to be run through.

[0035] After automatic compression detection has started, default actions to be applied by the control unit will initially be carried out. Thus, for instance, in the case of a diesel engine featuring common rail direct injection the rail pressure will be set to zero; this prevents any emergence of diesel fuel and its ingress into the combustion chambers, even if the fuel injectors are leaking.

[0036] In order to be able to carry out a meaningful leakage test of the combustion chambers, by means of which the compression pressure P_(zyl) can be stated in bar for each cylinder i, the constraints that must be complied with need to be specified precisely for each combustion engine. This includes for instance a minimum and a maximum crankshaft speed n during slow starter operation, a minimum temperature of the engine oil, a minimum battery voltage, as well as the suppression of the fuel supply. To this end, the combustion engine must be provided with sensors for determining oil temperature, battery voltage, and fuel supply. If one of the engine-specific constraints is outside an applicable threshold value, then the leakage testing of the combustion chambers will not be carried out, as this leakage testing could be too defective.

[0037] In order to test the constraints by means of the sensors and for the general conditioning of the combustion engine, the crankshaft will be made by the starter to go through two full rotations (720° crankshaft rotation angle=a single working cycle x of the combustion engine) before the compression pressure P_(zyl) is determined. This avoids any influences on engine speed determination due to the acceleration of the rotating and oscillating engine parts. Also, due to the oil pressure being built up, the lubrication of the combustion engine will cut in which again avoids any faults in engine speed determination.

[0038] If these constraints are not met, then the start of the automatic compression detection will be re-initiated.

[0039] If these constraints are met, the detection of crankshaft speeds n for determining the compression pressure P_(zyl) in the combustion chambers will cut in.

[0040] Following at least a further working cycle x of the combustion engine (=720° crankshaft rotation angle) each cylinder i will have run through all four working strokes, and 72 current crankshaft speeds n were determined. Such an actual speed curve is shown in FIG. 1. From the curve of the crankshaft speeds n it can be seen that, following the top dead center of each cylinder i, the crankshaft speed n shows a minimum which—on that curve—is then followed by a maximum. This engine shaft speed differential or this speed stroke Z_(i) of the crankshaft speed n, the difference from the maximum following the top dead center and the minimum directly following the top dead center of a cylinder i is used as a measure to determine whether there is any leakage in the combustion chambers.

[0041] In order to increase the evaluation precision, several working cycles x, e.g. three working cycles, will be recorded, with a mean value being derived from the values of the crankshaft speeds n of the respective same crankshaft rotation angle within the 720° working cycle x. Compliance with the constraint will be monitored continuously, and, if the actual value exceeds or falls below the threshold value of a constraint, this will cause a re-start of the automatic compression detection.

[0042] As a further constraint, it is possible to check here whether the deviation of the respective same values within the working cycle x of the combustion engine does not exceed an applicable threshold value.

[0043] For evaluation by the control unit, the cylinder i with the greatest speed stroke Z_(i) will be regarded as being the cylinder i with 100% compression pressure P_(zyl). A characteristic value K_(i) between 0% and 100% standardized to the speed stroke Z_(i) will be allocated to the other seven cylinders i of the combustion engine in line with that speed stroke Z_(i). The maximum compression pressure P_(zyl) of the nominal compression ratio of the combustion engine will be allocated to the cylinder i with 100% speed stroke Z_(i). The nominal compression ratio of the combustion engine is stored in the control unit. The percentage share of the maximum compression pressure P_(zyl) corresponding to their characteristic value K_(i) will be allocated to the other seven cylinders.

[0044] If the process is applied to a combustion engine with just a single cylinder i, the determined characteristic value K_(i) will be compared to a characteristic value K_(i) of cylinder i stored in the control unit, which value was determined following manufacture of the combustion engine.

[0045] In order to increase the evaluation precision even further, it is possible to correct the determined characteristic values K_(i) of cylinders i with regard to the averaged crankshaft speeds n and with regard to an engine temperature, for example the temperature of the engine oil.

[0046] To this end, the influence of the averaged crankshaft speeds n and the engine temperature on the characteristic values K_(i) will be determined, and stored as a characteristic curve f_((T)), f_((n)) in the control unit.

[0047] A further influencing quantity, which makes it more difficult to determine the characteristic values K_(i) of cylinders i correctly, is the cross-influencing among the cylinders i. This cross-influencing results from the fact that a cylinder i with a higher leakage will accelerate the crankshaft less powerfully on expansion of the compressed gas mixture than a cylinder i with a very low leakage. This causes the next cylinder i in the ignition sequence to have a lower speed stroke Z_(i) even if it is 100% proof against leaks. FIG. 1 shows this cross-influence in relation to the cylinder i=3 featuring a leakage and the next cylinder i=5 in the ignition sequence, which is 100% proof against leaks. Whilst the cylinders i=1, 6, 4, 7, 2, 8 feature a speed stroke Z_(i) value {circle over (1)} which corresponds to a 100% leak proof combustion chamber, the cylinder i=3 features a leakage and only supplies a speed stroke Z_(i) value {circle over (2)}. This causes the next cylinder i=5 in the ignition sequence to have a lower “accelerating effect” and shows—even if it is 100% proof against leaks—a speed stroke Z_(i) value {circle over (3)} which is lower than the speed stroke Z _(i) value {circle over (1)} of the 100% leak proof cylinders i.

[0048] Experiments have shown that it is sufficient to apply the correction only to the next cylinder i+1 in the ignition sequence. The influence on the next but one cylinder i+2 in the ignition sequence is negligibly low.

[0049] If, during leakage testing of the combustion chambers, the control unit determines a deviation in the characteristic values K_(i) of cylinders i, which exceeds an applicable threshold value, this can be indicated on the control panel and/or stored and retrieved again in a specialist workshop for diagnosis purposes. If necessary, whenever a leakage occurs in a combustion chamber the control unit can adapt the engine control system in a suitable fashion, for instance by a reduced firing of the relevant cylinder i. 

What is claimed is:
 1. Process for the cylinder-selective leakage testing of the combustion chambers in a combustion engine where engine shaft rotation angles are detected and current engine shaft speeds are determined during engine operation with suppressed combustion, characterized by the cyclical process steps: Determining the engine shaft speed differentials (Z₁) to be allocated to the cylinder (i) from the course of the current engine shaft speeds, forming a characteristic value (K_(i)) proportional to the value of the engine shaft speed differential (Z_(i)), allocating the characteristic value (K_(i)) to a compression pressure value (P_(zyl)), comparing the compression pressure (P_(zyl)) to a specified threshold value.
 2. Process according to claim 1 wherein, before engine shaft speeds are detected, a test for compliance with the constraints prerequisite for any leakage testing is carried out.
 3. Process according to claim 1 wherein the increase in the engine shaft speed following the top dead center of the cylinder (i) is utilized to determine the engine shaft speed differentials (Z_(i)) from the course of the engine shaft speeds.
 4. Process according to claim 3 wherein the engine shaft speed differentials (Z_(i)) are determined by determining the value of the maximum engine shaft speed differential after the top dead center of the cylinder (i) has been moved through minus the engine shaft speed directly following the top dead center of the same cylinder (i) for each cylinder (i) of the combustion engine.
 5. Process according to claim 4 wherein, as the characteristic value (K_(i)) of a cylinder (i), the percentage share of the engine speed differential (Z_(i)) of cylinder (i) is used in relation to the cylinder (i) with the maximum engine shaft speed differential (Z_(i)).
 6. Process according to claim 1 wherein, when the engine shaft speed is determined on a finite number of engine shaft rotation angles, two adjacent engine shaft rotation angles enclose a rotation angle segment, and wherein the resulting engine shaft speed of an engine shaft rotation angle represents the mean engine shaft speed of the rotation angle segment, and the engine shaft speed differentials (Z_(i)) of the cylinders (i) relative to the mean engine shaft speeds of the rotation angle segments are corrected in accordance with applicable characteristic curves (f_((n))).
 7. Process according to claim 1 wherein the engine shaft speed differentials (Z_(i)) determined for each cylinder are corrected relative to a combustion engine temperature in accordance with applicable characteristic curves (f_((T))).
 8. Process according to claim 1 wherein in the case of combustion engines with multiple cylinders (i) the engine shaft speed differential (Z_(i)) for the next cylinder (i) in the ignition sequence is corrected in relation to the engine shaft speed differential (Z_(i)) of the last cylinder (i) in the ignition sequence.
 9. Process according to claim 1 wherein the compression pressures (P_(zyl)) of the cylinders (i) and/or the engine shaft speed differentials (Z_(i)) and/or the characteristic values (K_(i)) are stored for comparative purposes after the combustion engine has been manufactured, repaired, or at any other intervals required.
 10. Process according to claim 1 , wherein the irregularities in the compression pressures (P_(zyl)) are shown.
 11. Process according to claim 1 wherein, before starting the detection of the current engine shaft speeds, the combustion engine runs through at least one working cycle (x).
 12. Process according to claim 1 wherein the crankshaft speed (n), the camshaft speed or the starter shaft speed are used as engine shaft speeds.
 13. Process according to claim 1 , wherein the process is used for combustion engines with just a single cylinder (i). 